1. Field of the Invention
The present invention relates to a piezoelectric composition and piezoelectric device widely used in areas such as piezoelectric sounders, piezoelectric sensors, piezoelectric actuators, piezoelectric transformers, and piezoelectric ultrasonic motors.
2. Description of the Related Art
Piezoelectric compositions have the effect of inducing strain when electric fields are applied to the piezoelectric compositions from outside (the effect of converting electrical energy into mechanical energy) and the effect of generating a charge on the surface when stresses are applied to the piezoelectric compositions from outside (the effect of converting mechanical energy into electrical energy). In recent years, the piezoelectric compositions have been widely used in various areas. For example, a piezoelectric composition such as lead zirconate titanate (Pb(Zr, Ti)O3, (PZT)) is excellent in micro-positioning and therefore is used to micro-position an optical system because the piezoelectric composition induces strain substantially in proportion to the voltage applied thereto on the order of 1×10−10 m/V and therefore is excellent in micro-positioning. In contrast, the piezoelectric composition is used as a sensor for reading micro-force or deformation because the piezoelectric composition generates a charge with a magnitude proportional to the stress applied to the piezoelectric composition or the deformation due to the stress. Furthermore, the piezoelectric composition is used as a piezoelectric transformer, an ultrasonic motor, or the like because the piezoelectric composition has excellent response and therefore can induce resonance in such a way that the piezoelectric composition or an elastic body joined to the piezoelectric composition is excited by applying an alternating-current electric field thereto.
As of now, most of piezoelectric compositions in practical use are a type of solid solution (PZT) containing PbZrO3—PbTiO3 (PZ-PT). Products meeting various needs are being developed by adding various auxiliary components or additives to such PZT-type piezoelectric compositions. Examples of the products include those which have a low mechanical quality factor (Qm) and a high piezoelectric constant (d) and which are used in actuators, required to have a large displacement in direct-current applications, for positioning and those which have a low piezoelectric constant (d) and a high mechanical quality factor (Qm) and which are suitable for use in ultrasonic wave-generators, such as ultrasonic motors, used in alternating-current applications.
Piezoelectric compositions other than PZT-type piezoelectric compositions are in practical use. Most of these piezoelectric compositions are solid solutions made of a lead-based perovskite composition such as lead magnesate niobate (Pb(Mg, Nb)O3, PMN).
However, lead-based piezoelectric compositions contain a large amount, about 60% to 70% by mass, of lead oxide, which has extremely high volatility at low temperature, as a major component. It is desired that the amount of lead oxide used is reduced in view of environmental concerns. Thus, if applications of piezoelectric ceramics and piezoelectric single-crystals are expanded in the future and therefore the amount of the piezoelectric ceramics and piezoelectric single-crystals used is increased, then the production of lead-free piezoelectric compositions will be an extremely important issue.
For example, barium titanate (BaTiO3) and bismuth layered ferroelectrics are known as piezoelectric compositions containing no lead at all. However, barium titanate has a low Curie temperature, 120° C., and loses its piezoelectricity at a temperature not lower than the Curie temperature thereof. Therefore, barium titanate is not suitable for practical use in consideration of bonding by soldering or on-vehicle applications. On the other hand, the bismuth layered ferroelectrics usually have a Curie temperature of 400° C. or higher and are excellent in thermal stability. However, the bismuth layered ferroelectrics have high crystallographic anisotropy; hence, it is difficult to polarize the bismuth layered ferroelectrics by ordinary polarization. Therefore, a technique, represented by a hot forging process or the like, for inducing spontaneous polarization by shear stress is necessary. However, the technique has a problem with productivity.
On the other hand, novel piezoelectric compositions such as sodium bismuth titanate-based compositions are recently investigated. For example, Japanese Patent No. 4177615 (hereinafter referred to as the patent document) discloses piezoelectric compositions containing sodium bismuth titanate or barium titanate.
However, the sodium bismuth titanate-based piezoelectric compositions are unsuitable for obtaining sufficient piezoelectric properties as compared to lead-based piezoelectric compositions. In particular, the sodium bismuth titanate-based piezoelectric compositions are insufficient in spontaneous polarization and piezoelectric constant. Therefore, the sodium bismuth titanate-based piezoelectric compositions need to be improved in piezoelectric properties.
In the case of adding an element for the purpose of enhancing piezoelectric properties, elements contained in a perovskite structure react with an added oxide to produce many secondary phases, resulting in difficulty in polarization.
In general, Bi-based perovskite compositions are likely to have a heterogeneous structure containing coarse grains with a size of more than 100 μm and fine grains with a size of several micrometers because crystal grains are likely to be extraordinarily grown during firing. Therefore, it is difficult to control the structure of the Bi-based perovskite compositions.
A piezoelectric ceramic disclosed in the patent document contains a major component that is a perovskite-type composite oxide represented by the general formula ABO3. In the general formula ABO3, A is one or more selected from the group consisting of Bi, Na, K, and Ba; B is Ti; and B (that is, Ti) is partly substituted with a trivalent element M(III). In the composition range disclosed in the patent document, sufficient piezoelectric properties are not obtained. Since the amount of Bi in an A-site is invariant even if the amount of the added trivalent element M(III) is increased by a method described in the patent document, the deliquescence of an excessive alkali metal in an A-site occurs. Therefore, heterophases may possibly be induced with a change in sintering temperature.